Read the sentence.

Someone sang a solo during the holiday concert.

The best reason to revise this sentence using the passive voice is that

During the holiday concert, someone sang a solo.

The best reason to revise this sentence using the passive voice is that the person uttering the sentence is not sure about who sang the solo.

Explanation:

The sentence above does not have a specific subject, the speaker has chosen to use the indefinte pronoun someone. Indefinite pronouns do not refer to any person, amoun, or thing in particular. As the singer is not mentioned at all, it would be better to put emphasis on the solo itself. This can be done by means of a passive construction. The sentence would be as follows: A solo was sang during the holiday.